A blood pump disclosed in U.S. Pat. No. 4,507,048 comprises an impeller being supported in the pump housing between two tip bearings, the blades of the impeller being arranged at the front of a central cone. On the back side of the central cone, there is a plate having a constant distance from the back wall of the pump housing. The profiling of the blades is similar to that of aircraft wings, and they have an angle of contact of about 15.degree.. The blades are covered by a cone envelope in which another cone envelope is arranged so that the impeller forms an altogether rotating partially hollow body wherein the blades are arranged.
From EP 0 451 376 A1, a blood pump is known wherein the impeller comprises a plane plate from which the blades project to the front, towards the inlet. The blades are slightly bent and their height decreases linearly outward. The impeller is attached to a shaft one end of which is supported in an extension of the pump housing. The front wall of the pump housing has a truncated configuration, and, with increasing radius, the back wall is set back.
Further, a blood pump is known from U.S. Pat. No. 4,589,822, wherein the impeller is fastened to a shaft which is also supported outside the pump housing. The impeller comprises linear blades whose height decreases linearly outwards. The front wall of the pump housing has a truncated configuration and, with the radius increasing, the back wall is set back. The blades only have an angle of contact of about 60.degree.. Outwards, they project beyond the plate.
From U.S. Pat. No. 4,984,972, a blood pump is known in which an impeller consisting of a plate with a plane upper surface and a conically extending lower surface is oscillatingly supported on a tip bearing. The height of the blades of the impeller linearly decreases radially outward, the blades terminating at the outer plate edge.
Centrifugal pumps for industrial applications are configured such that they have a high pump rate with low delivery pressure. On the contrary, blood pumps have to be configured for low pump rates and relatively high pressures. A problem with blood pumps is that they are subject to considerably varying operational conditions and that it has to be ensured that harm to blood is avoided. A blood pump, for example, is used for taking over the pump function of the heart of a patient during an operation. When a vasodilative medicine is administered to the patient, the fluid resistance of the patient body decreases and the pressure against which the blood pump has to feed decreases. Further, blood pumps can be used for fully taking over the heart function or for exerting a heart-supporting function only. Accordingly, a blood pump has to be capable of delivering varying quantities (by means of different speeds). Furthermore, a blood pump has to be configured such that it operates in the occurring wide application ranges with minimum blood disintegration. Blood disintegration happens, e.g., by local temperature rises of the blood pump in the support region of the impeller, but particularly by transverse stresses and shearing stresses to which the blood is exposed in the centrifugal pump. Such effects cause a disintegration of the blood due to hemolysis, thrombocytes being activated and aggregating. This may lead to perilous clot formations. Clot formations also form in dead water zones of insufficient flow through the pump housing.
An optimization of the flow conditions in blood pumps with the target to avoid any harm to blood cannot be achieved at present on the basis of calculations and theoretical considerations due to the various operational conditions to which a blood pump may be exposed. When designing a blood pump, the engineer is dependent, to a great extent, on empiric research.